The general subject matter of this invention relates to cooling channels for articles used at high temperatures, such as gas turbine engines. Some specific embodiments relate to methods for making and shaping the cooling channels.
A gas turbine engine includes a compressor, in which engine air is pressurized. The engine also includes a combustor, in which the pressurized air is mixed with fuel, to generate hot combustion gases. In a typical design (e.g., for aircraft engines or stationary power systems), energy is extracted from the gases in a high pressure turbine (HPT) which powers the compressor, and in a low pressure turbine (LPT). The low pressure turbine powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
The need for cooling systems in gas turbine engines is critical, since the engines usually operate in extremely hot environments. For example, the engine components are often exposed to hot gases having temperatures up to about 3800° F. (2093° C.), for aircraft applications, and up to about 2700° F. (1482° C.), for the stationary power generation applications. To cool the components exposed to the hot gases, these “hot gas path” components typically have both internal convection and external film cooling.
Many aspects of cooling circuits and features in various hot gas path components have been described in the art. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability, while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. For example, a series of internal cooling passages, or serpentine channels, may be formed in a hot gas path component. A cooling fluid may be provided to the serpentines from a plenum, and the cooling fluid may flow through the passages, cooling the hot gas path component substrate and coatings. While this type of cooling design may be effective in some cases, its use in other situations may result in comparatively low heat transfer rates and non-uniform component temperature profiles.
Microchannel cooling (as the feature is explained below) has the potential to significantly reduce cooling requirements by placing the cooling as close as possible to the heat zone, thus reducing the temperature difference between the hot side and cold side for a given heat transfer rate. However, current techniques for forming microchannels typically require the use of a sacrificial filler to keep the coating from being deposited within the microchannels (which usually need to be formed by machining). The sacrificial filler also supports the coating during deposition.
The particular path of a fluid through a cooling circuit is critical for ensuring that the coolant effectively draws heat away from regions subjected to the highest temperatures encountered during operation of a particular turbine engine or other device. As an example, a microchannel can be considered to be a three-dimensional region having various configurations, and including both channel sidewalls, along with some type of bottom surface. Each of the surfaces of the microchannel may be exposed to a different temperature profile during engine operation, depending in part on the curvature, roughness, and overall design of the cooling circuit. Providing more coolant than necessary to a region with a lower temperature profile would be inefficient, and may deprive other high-temperature surfaces with inadequate coolant. This is especially the case when the amount of coolant air available is limited. For example, in the case of a turbine engine, coolant air often needs to be bled off the engine compressor, and engine efficiency may suffer if too much coolant air is diverted to the cooling circuit.
Different techniques can be undertaken to influence the coolant flow characteristics within a cooling channel. As an example, a selected design of the cooling channel could be obtained during the casting of the particular component in which the channel is located. Moreover, machining techniques could be used to alter the depth and shape of a cooling channel that has already been formed. Examples of those processes are laser drilling, water jet cutting, and electro-discharge machining (EDM).
The machining techniques can be effective for beneficially altering the shape of a cooling channel. However, there are drawbacks associated with such methods. For example, they can be time-consuming, thereby increasing the cost of component fabrication. Furthermore, special care sometimes needs to be taken, to ensure that machining processes do not adversely affect the integrity of the component.
Thus, additional processes for modifying the shape of a coolant channel would be welcome in the art. The new processes should be capable of modifying the channel in a way that improves its heat transfer characteristics. They should also be capable of efficient implementation, and compatibility with other processes related to fabrication of the particular component. Moreover, shaping processes that also improved other characteristics of the coolant channel, such as the oxidation resistance of the channel surfaces, would be of considerable interest.